The present invention relates to a current reversal device, particularly a commutator, and to a process for production of such device. Such devices are employed in electric motors and current generators, for example, in tools powered by electricity, actuating drives, and fuel pumps.
Generic current reversal devices are disclosed, for example, in DE 41 37 400 C2. In this patent, a composite segment is stamped from a rolled or drawn copper strip and then is roller burnished, split a short distance or scored, and sprayed with a compression molding compound forming the outer cover of the commutator after settling. The bore in the outer cover must then be machined. The clasps of the commutator segments must be bent for fastening the ends of the windings. After an additional scaling or stripping procedure, the commutators undergo electrical testing and are then mounted on the engine shaft by force fitting.
DE 195 30 051 A1 discloses a plug-in commutator in which the commutator segments are inserted into an assembly cage and then sprayed with molded plastic material to form the outer cover. These operations are followed by additional machining and testing steps carried out to comply with the requirements set for accuracy of the geometric dimensions of the commutator and for the stability of the commutator.
DE-OS-2 352 155 discloses a commutator for a miniature electric motor and a process for manufacture of this motor. A desired number of commutator plates are fastened on a jacket surface of a core in specific sectors by an adhesive.
WO 95/14319 discloses a commutator and a process for its manufacture in which the segmented receiving means are undersized. The insulating elements and/or the segments have an elasticity such that the segments inserted into the segment receiving means are fastened both by form locking and by force locking. In addition, the segments may be caulked or cemented to the outer cover.
U.S. Pat. No. 3,819,967 shows a drum commutator in which the commutator segments are fastened to the cylindrical outer cover by cementing.
EP 0 361 860 A2 discloses a commutator in which the segments are fastened on the outer cover with an adhesive layer inserted between them. In addition, the segments are bent, at least on one end, to form a clasp. The segments are engaged in an incision in the outer cover which recedes radially from the circumferential surface of the outer cover and extends axially.
The metal segments are oxidized on the surface of their interior narrow side and then bonded to the ceramic body by being heated to the temperature required for production of a eutectic.
U.S. Pat. No. 5,629,576 discloses a planar commutator with commutator segments containing carbon. The segments are fastened to connection means of copper by an adhesive bonding agent. A segment is oriented toward the connection means by way of a recess in the segment containing carbon into which the bonding agent is also introduced.
Conventional commutators require a large number of production and testing steps in order to make it possible to guarantee the required degrees of accuracy and reliability.
Objects of the present invention are to provide a commutator exhibiting high accuracy with respect to its geometric dimensions and high long-term stability, while being simple to manufacture.
The segments may be fastened on the outer cover by fastening means mounted more or less between the outer cover and the segments. The outer cover is generally made from an electrically insulating material, particularly of a plastic such as a duroplastic, a thermoplastic, or of a ceramic material. Consideration may be given, as an alternative, to a metal outer cover such as one of aluminum whose surface is provided with a preferably electrically insulating coating such as a coating of varnish or a layer of metal oxide produced by oxidation of the metal outer cover. The outer cover may also be built up in two or more layers, and especially, may have a flexible internal hub surrounded by a temperature-stable outer shell on which the segments may be fastened. The flexible internal hub provides the necessary force fitting for mounting the commutator on an engine shaft. The commutator segments generally comprise copper or a copper compound. Consideration may be given also to other materials in keeping with the requirements set for conductivity, temperature stability, and chemical resistance. The fastening means is preferably applied in layers between the segments and the outer cover, and may be applied prior to fastening on the outer cover and/or the segments.
The outer cover and the segments have interacting means for positioning and orienting the segments relative to the outer cover. They may be made in the form of punctiform, linear, or planiform projections and matching recesses on the outer cover or on the segments. The segments, for example, may have studlike projections which may be inserted into matching grooves oriented parallel to the axis of rotation on the circumference of the outer cover. In the case of a planar commutator, radially oriented studs on a frontal surface may fit into corresponding recesses or grooves on the pertinent segment; or the cylindrical outer cover may have on the edge on its front side an axially projecting and preferably circumferentially annular continuous projection to ensure centering of the disk segment to, be fastened on the front surface. The disk segment may then be divided into individual commutator segments electrically insulated from each other.
If the fastening means is a layer of adhesive, this layer may be filled with suitable additive meeting electric and/or thermal requirements. The thermal coefficient of linear expansion of the adhesive layer, for example, can be reduced by a ceramic filler. If required, an electrically conductive metal filler, such as one based on Ag, Cu, or Ni may be used to establish a connection between the segment and the outer cover, for example, if the outer cover for a planar commutator comprises electrically conductive segmental connecting lines. The fillers may in particular define a distance between the commutator segments and the outer cover, and thus, the thickness of the adhesive layer, preferably by means of spherical fillers, especially ones in the form of glass or ceramic spheres. The layer thickness ranges, for example, from 20 to 250 xcexcm, and preferably from 50 to 100 xcexcm. The layer thickness may also be determined in advance by means of spacers, preferably in one piece, made up of the outer cover or the commutator segments, as for example by punctate, linear, or planiform projections. The adhesive to be selected or treated will absorb as little moisture as possible after setting, will form a permanent strong bond with copper in particular and is dimensionally stable when subjected to mechanical and/or thermal stress.
If the fastener is a soldered or welded layer, the commutator possesses particularly high temperature stability and chemical resistance. Consideration is given in this case preferably to low melting soft, hard, or glass solders, such as low melting lead/tin solders or glass solders with a high lead oxide content. An especially low bonding temperature is obtained by ultrasound or friction welding.
If the segments and the outer cover have force fitted interacting anchor and receiving means, the segments may be inserted into the outer cover. The spring loaded anchor and receiving means operating in opposition to each other form a sufficiently stable clamp connection. In addition, the segments on the outer cover may be positioned by the bonding means. It is also possible, however, to dispense with an additional bonding agent and to position the segments exclusively with anchor and receiving means forming a clamp connection. Both the segments and the outer cover may have only anchor means or only receiving means or a combination of anchor and receiving means. The sole essential requirement is that anchor or receiving means of the segment interact with receiving or anchor means of the outer cover.
It is especially advantageous to be able to insert or clip the segments radially into the outer cover, as for example in the case of a drum commutator, or to insert them axially into a frontal surface of the outer cover, in the case of a planar commutator, for example.
It is also advantageous for the positioning and orienting of the segments to take place simultaneously with insertion of the segments into the outer cover by means of the anchor or receiving means. Means for positioning and orienting extend preferably parallel to the axis of rotation along a peripheral surface or radially to the axis of rotation along a frontal surface of the outer cover. All suitable configurations, in particular studs which are triangular, rectangular, hemispherical, or swallow-tailed in cross-section, may be considered as positioning and orienting means. Cross-sectional shapes which widen out in depth, and especially ones provided with a point for ease of insertion, are particularly well suited for anchoring.
In the process of the present invention, segments are fastened to the outer cover by means of a bonding agent mounted more or less between the outer cover and the segments. For example, the outer cover as a whole may be immersed in a bonding means bath before the segments are fastened. Alternatively or in addition, at least the surface of the segments facing the outer cover may be provided with the bonding means. Optionally, the surfaces of the outer cover and/or of the segments are to be cleaned and/or provided with a bonding agent before the fastener is applied. The surfaces may by preference be conditioned in a vacuum process, such as in an ion or plasma vacuum process. Surface treatment may also be carried out to achieve adequate aging and corrosion resistance of the fastening layer to stresses during subsequent use and/or uniform wetting means with the fastener. Secondary treatment of the bonded joint is also advantageous in cementing, in particular, to prevent corrosion and/or infiltration, and so reduction of the strength of the fastening.
Means for positioning and orienting the segments are provide whose shape is such that positioning and orienting take place automatically as the segments are delivered. For example grooves triangular in cross-section in the outer cover can be used, into which studs of the segments of matching cross-section are introduced. In this case, the fastening means may be introduced into the groove, for example, as a line of adhesive, before delivery. As the segments are subsequently delivered, the fastening means is displaced to form a flat bonding layer between outer cover and segment.
If a clamp connection is established between anchor and receiving means, fastening means introduced between the outer cover and the segments may be omitted. In this instance, fastening is effected exclusively by means of anchor and receiving means forming part of a clamp connection.
If the fastening means is provided, an adhesive, solder, or weld layer is to be considered in particular for this purpose. The maximum temperature in additional treatment may be around 300xc2x0 C. for a brief period. Setting of an adhesive layer should always take place at the lowest possible temperature, for example, in the temperature range from 50 to 250xc2x0 C., preferably from 170 to 200xc2x0 C.
If the segments are delivered to the outer cover sequentially, this may be effected by step-by-step rotation of the outer cover around its axis of rotation and application of the segments piece by piece, or by rolling of the outer cover onto the segments interconnected in a strip, for example. In the case of step-by-step delivery, the connection between the outer cover and the pertinent segment may be established either immediately after delivery or as the final step for all segments delivered together, for example, by surrounding the outer cover with segments in place with compression and/or heating tongs.
If all segments are delivered to the outer cover simultaneously, this may be accomplished with a suitable compression and/or heating tool. Following delivery, the tool ensures mechanically reliable fastening of the segments to the outer cover. It may be effected, for example, by pressing the segments into the outer cover, in particular by pressing the anchor and receiving means together and/or by heating the segments to the point of melting of the fastening means and production of a bonding layer.